Primers (a/k/a detonators) are the means used to detonate an explosive charge. These devices consist of a primary explosive component initiated by stab (i.e.--friction) or impact, an intermediate explosive composition which is set off by the primer composition, and a base charge of secondary explosive such as RDX or HMX to provide the desired explosive output. These primers are ordinarily coupled with booster charges and a propellant to form an explodable ignition chain.
One of the common low input energy primers/detonators is an M55 Detonator, which is extensively used in ordnance for anti-personnel and anti-vehicular munition systems. The make up of this detonator consists of:
(a) A primary explosive composition containing basic lead styphnate, dextrinated lead azide, antimony sulfide, barium nitrate, and tetracene;
(b) An intermediate explosive charge of RD 1333 lead azide; and
(c) RDX as secondary explosive.
The M55 detonator is set off by stab action and shows a sensitivity of about 0.80 inch-oz at 99.99% reliability and 95% confidence level. In this detonator system, while basic lead styphnate and dextrinated lead azide serve as the main primary explosives, barium nitrate serves as a supplier of oxygen to the system and the antimony sulfide serves as a fuel and mechanical sensitizer because of its high melting point. But it is tetracene that plays a unique and important role. It is a chemical sensitizer possessing properties which makes the system function at an input sensitivity or energy below
Primer/detonator systems are employed in a variety of applications. Some of the more obvious applications are in weapon systems, razing buildings, and in creating excavations. However, such charges are also utilized in not so obvious applications such as automobile airbag systems. Explosive charges are uniquely suited for these systems because the explosions instantaneously generate a large volume of gas which fills the airbag. It is absolutely necessary that the airbags be filled instantaneously, as the time between an automobile crash and the passenger's impact with the windshield or dashboard is no more than a fraction of a second.
A typical explosive charge utilized in an airbag system is comprised of a primer/detonator (discussed below), a booster charge consisting of boron and potassium nitrate, and a propellant containing sodium azide, such as that disclosed in U.S. Pat. No. 3,947,300.
Since the function of the primer/detonator is to initiate the explosive reaction, the primer must possess some means of commencing the ignition of the chain. This means is known as the sensitizer, which may take either a chemical or mechanical form. As indicated above, one of the best chemical sensitizers is tetracene because of its excellent input sensitivities.
When the M55 primer is detonated, it sends off shock waves which while igniting the intermediate charge, shatters the cup containing the intermediate charge, resulting in unreliable ignition of the propellant charge. Therefore, it would be advantageous to provide a primer/detonator which may be used to ignite an airbag system which does not generate shock waves for igniting the system.